This invention relates to the preparation of maleic anhydride by catalytic oxidation of n-butane or other hydrocarbons having at least four carbon atoms in a straight chain, and more particularly to an improved process in which carryover of catalyst fines in the reaction gases exiting the catalytic reaction chamber is prevented.
Maleic anhydride is produced commercially by catalytic oxidation of n-butane, butenes or other hydrocarbons having at least four carbon atoms in a straight chain. Catalysts for the reaction have active sites comprising phosphorus vanadium oxides. Particularly preferred processes use a tubular reactor containing a fixed bed of phosphorus vanadium oxide catalyst shaped bodies, for example, cylinders, spheres, saddles, trilobes, etc., typically having a principal dimension of at least one eighth of an inch. Most commonly, the fixed catalyst bed is packed into the tubes of a shell and tube heat exchanger, and a salt bath is circulated through the shell of the exchanger for removal of the heat generated by the exothermic oxidation reaction. The reaction gases exiting the reactor contain maleic anhydride product, inert gases, water and other by products including acetic acid, acrylic acid, carbon monoxide and carbon dioxide.
Maleic anhydride is recovered from the reaction product gases by absorption in water or, as described for example in U.S. Pat. No. 4,118,403, by absorption in an organic solvent such as phthalate ester, preferably an alkyl phthalate. Dialkyl phthalates such as dibutyl phthalate have been used commercially for this purpose. Rich solvent exiting the absorber is stripped under vacuum for recovery of maleic anhydride from the solvent, and lean solvent is recycled to the absorber for further recovery of maleic anhydride from the reaction gases.
Over an extended course of operations, solvents such as dibutyl phthalate are observed to become contaminated with high molecular weight materials, commonly referred to as "tars." As described in U.S. Pat. No. 4,118,403, tars can be eliminated from the system by distilling the solvent, leaving a residue of high tar content which is purged from the system. Provision and operation of solvent distillation facilities adds to the capital and operating cost of a maleic anhydride manufacturing facility. To eliminate this operation, or at least minimize the energy consumed in solvent distillation and the requisite investment in distillation facilities, processes have been developed for extraction of tars from the recycle solvent stream. Such processes are described in U.S. Pat. No. 5,631,387.
While tar extraction offers a signal improvement in a maleic anhydride solvent absorption system, the extraction system involves its own capital and operating costs. Tar formation consumes absorption solvent regardless of how effectively the tars are removed; and the purge of tars through either distillation or extraction inevitably carries out some fraction of unhydrolyzed solvent. Maleic anhydride manufacturing operations would be further optimized, and the costs associated with solvent absorption further minimized, if the rate of tar formation were substantially reduced in the first instance.
Shaped bodies comprising phosphorus vanadium oxide ("VPO") catalysts are relatively soft and frangible. During maleic anhydride manufacturing operations, such catalyst have been observed to deteriorate, causing alterations of flow characteristics in the tubes of the maleic anhydride reactor. Inspection of the reactor during plant turnarounds sometimes reveals a significant loss of catalyst from some or all of the tubes. In exceptional circumstances, tubes can potentially be emptied of catalyst. Loss of catalyst not only reduces productivity of the reactor but also adversely affects yields on n-butane or other starting material. Yield loss may be aggravated by the passage of unreacted hydrocarbon feed gas through tubes which have been emptied of catalyst or suffered substantial catalyst loss.